DDR RAM is twice as fast as SDRAM. This is because DDR RAM transfer data on both the rising and falling edge of the clock whereas SDRAM only transfer data on the rising of the cycle. So, DDR RAM transfers twice the amount of data per clock cycle and therefore is referred to as DDR (Double Data Rate).

PC200 (200Mhz) and PC266 (266Mhz) are the current rating designations according to JEDEC standards. However, the RAM industry refers to PC200 as PC1600 and PC266 as PC2100. PC1600 DDR memory can transfer at speeds up to 1600MB/sec and PC2100 DDR memory can transfer at speeds up to 2133MB/sec. Also, DDR memory consumes less power than SDRAM at 2.5V instead of 3.3V, which helps to reduce the amount of heat that is produced. This helps extend the battery life in portable devices such as laptop computers.

DDR RAM has actually been available for quite some time now. It's taken a while for it to become available to general consumers because demand for it is just beginning to grow. You cannot replace SDRAM with DDR RAM due to the physical difference (168-pin with standard DIMMs and 184-pin with DDR RAM) and power requirements. There are also two notches on each end of the RAM stick whereas there is only one per end on SDRAM.

One of the first companies to implement the use of DDR RAM was nVIDIA with their line of high performace GeForce 3D video cards. Now, manufactures such as Asus have begun integrating it into their motherboards and computer manfucturers are releasing new computer systems with this technology. DDR RAM is now available in the form of a DIMM, a memory module which can be easily inserted into a memory slot to add memory to your system. The good news is that now that it is more commonly used, it has also become less expensive. You can even purchase it directly from manufacturers such as Micron's Crucial Technology division.

Additional information about DDR standards can be found at Crucial's DDR Information page, TeamDDR and JEDEC.

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